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Thanos Coding Style Guide

This document details the official style guides for the various languages we use in the Thanos project. Feel free to familiarize yourself with and refer to this document during code reviews. If something in our codebase does not match the style, it means it was missed or it was written before this document. Help wanted to fix it! (:

Generally, we care about:

Readability, so low Cognitive Load.
Maintainability. We avoid the code that surprises.
Performance only for critical path and without compromising readability.
Testability. Even if it means some changes to the production code, like timeNow func() time.Time mock.
Consistency: If some pattern repeats, it means fewer surprises.

Some style is enforced by our linters and is covered in separate smaller sections. Please look there if you want to embrace some of the rules in your own project! For Thanos developers, we recommend reading sections about rules to manually apply during development. Some of those are currently impossible to detect with linters. Ideally, everything would be automated. (:

Go

For code written in Go we use the standard Go style guides (Effective Go, CodeReviewComments) with a few additional rules that make certain areas stricter than the standard guides. This ensures even better consistency in modern distributed system databases like Thanos, where reliability, performance, and maintainability are extremely important.

Development / Code Review

In this section, we will go through rules that on top of the standard guides that we apply during development and code reviews.

NOTE: If you know that any of those rules can be enabled by some linter, automatically, let us know! (:

Reliability

The coding style is not purely about what is ugly and what is not. It's mainly to make sure programs are reliable for running on production 24h per day without causing incidents. The following rules are describing some unhealthy patterns we have seen across the Go community that are often forgotten. Those things can be considered bugs or can significantly increase the chances of introducing a bug.

Defers: Don't Forget to Check Returned Errors

It's easy to forget to check the error returned by a Close method that we deferred.

f, err := os.Open(...)
if err != nil {
    // handle..
}
defer f.Close() // What if an error occurs here?

// Write something to file... etc.

Unchecked errors like this can lead to major bugs. Consider the above example: the *os.File Close method can be responsible for actually flushing to the file, so if an error occurs at that point, the whole write might be aborted! 😱

Always check errors! To make it consistent and not distracting, use our runutil helper package, e.g.:

// Use `CloseWithErrCapture` if you want to close and fail the function or
// method on a `f.Close` error (make sure the `error` return argument is
// named as `err`). If the error is already present, `CloseWithErrCapture`
// will append (not wrap) the `f.Close` error if any.
defer runutil.CloseWithErrCapture(&err, f, "close file")

// Use `CloseWithLogOnErr` if you want to close and log error on `Warn`
// level on a `f.Close` error.
defer runutil.CloseWithLogOnErr(logger, f, "close file")

Avoid 🔥

func writeToFile(...) error {
    f, err := os.Open(...)
    if err != nil {
        return err
    }
    defer f.Close() // What if an error occurs here?

    // Write something to file...
    return nil
}

Better 🤓

func writeToFile(...) (err error) {
    f, err := os.Open(...)
    if err != nil {
        return err
    }
    // Now all is handled well.
    defer runutil.CloseWithErrCapture(&err, f, "close file")

    // Write something to file...
    return nil
}

Exhaust Readers

One of the most common bugs is forgetting to close or fully read the bodies of HTTP requests and responses, especially on error. If you read the body of such structures, you can use the runutil helper as well:

defer runutil.ExhaustCloseWithLogOnErr(logger, resp.Body, "close response")

Avoid 🔥

resp, err := http.Get("http://example.com/")
if err != nil {
    // handle...
}
defer runutil.CloseWithLogOnErr(logger, resp.Body, "close response")

scanner := bufio.NewScanner(resp.Body)
// If any error happens and we return in the middle of scanning
// body, we can end up with unread buffer, which
// will use memory and hold TCP connection!
for scanner.Scan() {

Better 🤓

resp, err := http.Get("http://example.com/")
if err != nil {
    // handle...
}
defer runutil.ExhaustCloseWithLogOnErr(logger, resp.Body, "close response")

scanner := bufio.NewScanner(resp.Body)
// If any error happens and we return in the middle of scanning body,
// defer will handle all well.
for scanner.Scan() {

Avoid Globals

No globals other than const are allowed. Period. This means also, no init functions.

Never Use Panics

Never use them. If some dependency uses it, use recover. Also, consider avoiding that dependency. 🙈

Avoid Using the `reflect` or `unsafe` Packages

Use those only for very specific, critical cases. Especially reflect tend to be very slow. For testing code, it's fine to use reflect.

Avoid variable shadowing

Variable shadowing is when you use the same variable name in a smaller scope that "shadows". This is very dangerous as it leads to many surprises. It's extremely hard to debug such problems as they might appear in unrelated parts of the code. And what's broken is tiny : or lack of it.

Avoid 🔥

    var client ClientInterface
    if clientTypeASpecified {
        // Ups - typo, should be =`
        client, err := clienttypea.NewClient(...)
        if err != nil {
            // handle err
        }
        level.Info(logger).Log("msg", "created client", "type", client.Type)
    } else {
        // Ups - typo, should be =`
         client, err := clienttypea.NewClient(...)
         level.Info(logger).Log("msg", "noop client will be used", "type", client.Type)
    }

    // In some further deeper part of the code...
    resp, err := client.Call(....) // nil pointer panic!

Better 🤓

    var client ClientInterface = NewNoop(...)
    if clientTypeASpecified {
        c, err := clienttypea.NewClient(...)
        if err != nil {
            // handle err
        }
        client = c
    }
    level.Info(logger).Log("msg", "created client", "type", c.Type)

    resp, err := client.Call(....)

This is also why we recommend to scope errors if you can:

    if err := doSomething(); err != nil {
        // handle err
    }

While it's not yet configured, we might think consider not permitting variable shadowing with golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow in future. There was even Go 2 proposal for disabling this in the language itself, but was rejected:

Similar to this problem is the package name shadowing. While it is less dangerous, it can cause similar issues, so avoid package shadowing if you can.

Performance

After all, Thanos system is a database that has to perform queries over terabytes of data within human friendly response times. This might require some additional patterns in our code. With those patterns, we try to not sacrifice the readability and apply those only on the critical code paths.

Keep in mind to always measure the results. The Go performance relies on many hidden things and tweaks, so the good micro benchmark, following with the real system load test is in most times required to tell if optimization makes sense.

Pre-allocating Slices and Maps

Try to always preallocate slices and map. If you know the number of elements you want to put apriori, use that knowledge! This significantly improves the latency of such code. Consider this as micro optimization, however, it's a good pattern to do it always, as it does not add much complexity. Performance wise, it's only relevant for critical, code paths with big arrays.

NOTE: This is because, in very simple view, the Go runtime allocates 2 times the current size. So if you expect million of elements, Go will do many allocations on append in between instead of just one if you preallocate.

Avoid 🔥

func copyIntoSliceAndMap(biggy []string) (a []string, b map[string]struct{})
    b = map[string]struct{}{}

    for _, item := range biggy {
        a = append(a, item)
        b[item] = struct{}
    }
}

Better 🤓

func copyIntoSliceAndMap(biggy []string) (a []string, b map[string]struct{})
    b = make(map[string]struct{}, len(biggy))
    a = make([]string, len(biggy))

    // Copy will not even work without pre-allocation.
    copy(a, biggy)
    for _, item := range biggy {
        b[item] = struct{}
    }
}

Reuse arrays

To extend the above point, there are cases where you don't need to allocate new space in memory all the time. If you repeat the certain operation on slices sequentially and you just release the array on every iteration, it's reasonable to reuse the underlying array for those. This can give quite enormous gains for critical paths. Unfortunately, currently there is no way to reuse the underlying array for maps.

NOTE: Why you cannot just allocate slice and release and in new iteration allocate and release again etc? Go should know it has available space and just reuses that no? (: Well, it's not that easy. TL;DR is that Go Garbage Collection runs periodically or on certain cases (big heap), but definitely not on every iteration of your loop (that would be super slow). Read more in details here.

Avoid 🔥

var messages []string
for _, msg := range recv {
    messages = append(messages, msg)

    if len(messages) > maxMessageLen {
        marshalAndSend(messages)
        // This creates new array. Previous array
        // will be garbage collected only after
        // some time (seconds), which
        // can create enormous memory pressure.
        messages = []string
    }
}

Better 🤓

var messages []string
for _, msg := range recv {
    messages = append(messages, msg)

    if len(messages) > maxMessageLen {
        marshalAndSend(messages)
        // Instead of new array, reuse
        // the same, with the same capacity,
        // just length equals to zero.
        messages = messages[:0]
    }
}

Readability

The part that all Gophers love ❤️ How to make code more readable?

For the Thanos Team, readability is about programming in a way that does not surprise the reader of the code. All the details and inconsistencies can distract or mislead the reader, so every character or newline might matter. That's why we might be spending more time on every Pull Requests' review, especially in the beginning, but for a good reason! To make sure we can quickly understand, extend and fix problems with our system.

Keep the Interface Narrow; Avoid Shallow Functions

This is connected more to the API design than coding, but even during small coding decisions it matter. For example how you define functions or methods. There are two general rules:

Simpler (usually it means smaller) interfaces are better. This might mean a smaller, simpler function signature as well as fewer methods in the interfaces. Try to group interfaces based on functionality to expose at max 1-3 methods if possible.

Avoid 🔥

// Compactor aka: The Big Boy. Such big interface is really useless ):
type Compactor interface {
    Compact(ctx context.Context) error
    FetchMeta(ctx context.Context) (metas map[ulid.ULID]*metadata.Meta, partial map[ulid.ULID]error, err error)
    UpdateOnMetaChange(func([]metadata.Meta, error))
    SyncMetas(ctx context.Context) error
    Groups() (res []*Group, err error)
    GarbageCollect(ctx context.Context) error
    ApplyRetentionPolicyByResolution(ctx context.Context, logger log.Logger, bkt objstore.Bucket) error
    BestEffortCleanAbortedPartialUploads(ctx context.Context, bkt objstore.Bucket)
    DeleteMarkedBlocks(ctx context.Context) error
    Downsample(ctx context.Context, logger log.Logger, metrics *DownsampleMetrics, bkt objstore.Bucket) error
}

Better 🤓

// Smaller interfaces with a smaller number of arguments allow functional grouping, clean composition and clear testability.
type Compactor interface {
    Compact(ctx context.Context) error

}

type Downsampler interface {
    Downsample(ctx context.Context) error
}

type MetaFetcher interface {
    Fetch(ctx context.Context) (metas map[ulid.ULID]*metadata.Meta, partial map[ulid.ULID]error, err error)
    UpdateOnChange(func([]metadata.Meta, error))
}

type Syncer interface {
    SyncMetas(ctx context.Context) error
    Groups() (res []*Group, err error)
    GarbageCollect(ctx context.Context) error
}

type RetentionKeeper interface {
    Apply(ctx context.Context) error
}

type Cleaner interface {
    DeleteMarkedBlocks(ctx context.Context) error
    BestEffortCleanAbortedPartialUploads(ctx context.Context)
}

It's better if you can hide more unnecessary complexity from the user. This means that having shallow function introduce more cognitive load to understand the function name or navigate to implementation to understand it better. It might be much more readable to inline those few lines directly on the caller side.

Avoid 🔥

    // Some code...
    s.doSomethingAndHandleError()

    // Some code...
}

func (s *myStruct) doSomethingAndHandleError() {
    if err := doSomething(); err != nil {
        level.Error(s.logger).Log("msg" "failed to do something; sorry", "err", err)
    }
}

Better 🤓

    // Some code...
    if err := doSomething(); err != nil {
        level.Error(s.logger).Log("msg" "failed to do something; sorry", "err", err)
    }

    // Some code...
}

This is a little bit connected to There should be one-- and preferably only one --obvious way to do it and DRY rules. If you have more ways of doing something than one, it means you have a wider interface, allowing more opportunities for errors, ambiguity and maintenance burden.

Avoid 🔥

// We have here SIX potential ways the caller can get an ID. Can you find all of them?

type Block struct {
    // Things...
    ID ulid.ULID

    mtx sync.Mutex
}

func (b *Block) Lock() {  b.mtx.Lock() }

func (b *Block) Unlock() {  b.mtx.Unlock() }

func (b *Block) ID() ulid.ULID {
    b.mtx.Lock()
    defer b.mtx.Unlock()
    return b.ID
}

func (b *Block) IDNoLock() ulid.ULID {  return b.ID }

Better 🤓

type Block struct {
    // Things...

    id ulid.ULID
    mtx sync.Mutex
}

func (b *Block) ID() ulid.ULID {
    b.mtx.Lock()
    defer b.mtx.Unlock()
    return b.id
}

Use Named Return Parameters Carefully

It's OK to name return parameters if the types do not give enough information about what function or method actually returns. Another use case is when you want to define a variable, e.g. a slice.

IMPORTANT: never use naked return statements with named return parameters. This compiles but it makes returning values implicit and thus more prone to surprises.

Clean Defer Only if Function Fails

There is a way to sacrifice defer in order to properly close all on each error. Repetition makes it easier to make an error and forget something when changing the code, so on-error deferring is doable:

Avoid 🔥

func OpenSomeFileAndDoSomeStuff() (*os.File, error) {
    f, err := os.OpenFile("file.txt", os.O_RDONLY, 0)
    if err != nil {
        return nil, err
    }

    if err := doStuff1(); err != nil {
        runutil.CloseWithErrCapture(&err, f, "close file")
        return nil, err
    }
    if err := doStuff2(); err != nil {
        runutil.CloseWithErrCapture(&err, f, "close file")
        return nil, err
    }
    if err := doStuff232241(); err != nil {
        // Ups.. forgot to close file here.
        return nil, err
    }
    return f, nil
}

Better 🤓

func OpenSomeFileAndDoSomeStuff() (f *os.File, err error) {
    f, err = os.OpenFile("file.txt", os.O_RDONLY, 0)
    if err != nil {
        return nil, err
    }
    defer func() {
        if err != nil {
             runutil.CloseWithErrCapture(&err, f, "close file")
        }
    }

    if err := doStuff1(); err != nil {
        return nil, err
    }
    if err := doStuff2(); err != nil {
        return nil, err
    }
    if err := doStuff232241(); err != nil {
        return nil, err
    }
    return f, nil
}

Explicitly Handle Returned Errors

Always handle returned errors. It does not mean you cannot "ignore" the error for some reason, e.g. if we know implementation will not return anything meaningful. You can ignore the error, but do so explicitly:

Avoid 🔥
someMethodThatReturnsError(...)
Better 🤓
_ = someMethodThatReturnsError(...)

The exception: well-known cases such as level.Debug|Warn etc and fmt.Fprint*

Avoid Defining Variables Used Only Once.

It's tempting to define a variable as an intermittent step to create something bigger. Avoid defining such a variable if it's used only once. When you create a variable the reader expects some other usage of this variable than one, so it can be annoying to every time double check that and realize that it's only used once.

Avoid 🔥

    someConfig := a.GetConfig()
    address124 := someConfig.Addresses[124]
    addressStr := fmt.Sprintf("%s:%d", address124.Host, address124.Port)

    c := &MyType{HostPort: addressStr, SomeOther: thing}
    return c

Better 🤓

    // This variable is required for potentially consistent results. It is used twice.
    someConfig := a.FetchConfig()
    return &MyType{
        HostPort:  fmt.Sprintf("%s:%d", someConfig.Addresses[124].Host, someConfig.Addresses[124].Port),
        SomeOther: thing,
    }

Only Two Ways of Formatting Functions/Methods

Prefer function/method definitions with arguments in a single line. If it's too wide, put each argument on a new line.

Avoid 🔥

func function(argument1 int, argument2 string,
    argument3 time.Duration, argument4 someType,
    argument5 float64, argument6 time.Time,
) (ret int, err error) {

Better 🤓

func function(
    argument1 int,
    argument2 string,
    argument3 time.Duration,
    argument4 someType,
    argument5 float64,
    argument6 time.Time,
) (ret int, err error)

This applies for both calling and defining method / function.

NOTE: One exception would be when you expect the variadic (e.g. ...string) arguments to be filled in pairs, e.g:

level.Info(logger).Log(
    "msg", "found something epic during compaction; this looks amazing",
    "compNumber", compNumber,
    "block", id,
    "elapsed", timeElapsed,
)

Control Structure: Prefer early returns and avoid `else`

In most of the cases, you don't need else. You can usually use continue, break or return to end an if block. This enables having one less indent and better consistency so code is more readable.

Avoid 🔥
for _, elem := range elems { if a == 1 { something[i] = "yes" } else something[i] = "no" } }
Better 🤓
for _, elem := range elems { if a == 1 { something[i] = "yes" continue } something[i] = "no" }

Wrap Errors for More Context; Don't Repeat "failed ..." There.

We use pkg/errors package for errors. We prefer it over standard wrapping with fmt.Errorf + %w, as errors.Wrap is explicit. It's easy to by accident replace %w with %v or to add extra inconsistent characters to the string.

Use pkg/errors.Wrap to wrap errors for future context when errors occur. It's recommended to add more interesting variables to add context using errors.Wrapf, e.g. file names, IDs or things that fail, etc.

NOTE: never prefix wrap messages with wording like failed ... or error occurred while.... Just describe what we wanted to do when the failure occurred. Those prefixes are just noise. We are wrapping error, so it's obvious that some error occurred, right? (: Improve readability and consider avoiding those.

Avoid 🔥
if err != nil { return fmt.Errorf("error while reading from file %s: %w", f.Name, err) }
Better 🤓
if err != nil { return errors.Wrapf(err, "read file %s", f.Name) }

Use the Blank Identifier `_`

Blank identifiers are very useful to mark variables that are not used. Consider the following cases:

// We don't need the second return parameter.
// Let's use the blank identifier instead.
a, _, err := function1(...)
if err != nil {
    // handle err
}

// We don't need to use this variable, we
// just want to make sure TypeA implements InterfaceA.
var _ InterfaceA = TypeA

// We don't use context argument; let's use the blank
// identifier to make it clear.
func (t *Type) SomeMethod(_ context.Context, abc int) error {

Rules for Log Messages

We use go-kit logger in Thanos. This means that we expect log lines to have a certain structure. Structure means that instead of adding variables to the message, those should be passed as separate fields. Keep in mind that all log lines in Thanos should be lowercase (readability and consistency) and all struct keys are using camelCase. It's suggested to keep key names short and consistent. For example, if we always use block for block ID, let's not use in the other single log message id.

Avoid 🔥

level.Info(logger).Log("msg", fmt.Sprintf("Found something epic during compaction number %v. This looks amazing.", compactionNumber),
 "block_id", id, "elapsed-time", timeElapsed)

Better 🤓

level.Info(logger).Log("msg", "found something epic during compaction; this looks amazing", "compNumber", compNumber,
"block", id, "elapsed", timeElapsed)

Additionally, there are certain rules we suggest while using different log levels:

level.Info: Should always have msg field. It should be used only for important events that we expect to happen not too often.
level.Debug: Should always have msg field. It can be a bit more spammy, but should not be everywhere as well. Use it only when you want to really dive into some problems in certain areas.
level.Warn: Should have either msg or err or both fields. They should warn about events that are suspicious and to investigate but the process can gracefully mitigate it. Always try to describe how it was mitigated, what action will be performed e.g. value will be skipped
level.Error: Should have either msg or err or both fields. Use it only for a critical event.

Comment Necessary Surprises

Comments are not the best. They age quickly and the compiler does not fail if you will forget to update them. So use comments only when necessary. And it is necessary to comment on code that can surprise the user. Sometimes, complexity is necessary, for example for performance. Comment in this case why such optimization was needed. If something was done temporarily add TODO(<github name>): <something, with GitHub issue link ideally>.

Testing

Table Tests

Use table-driven tests that use t.Run for readability. They are easy to read and allows to add a clean description of each test case. Adding or adapting test cases is also easier.

Avoid 🔥

host, port, err := net.SplitHostPort("1.2.3.4:1234")
testutil.Ok(t, err)
testutil.Equals(t, "1.2.3.4", host)
testutil.Equals(t, "1234", port)

host, port, err = net.SplitHostPort("1.2.3.4:something")
testutil.Ok(t, err)
testutil.Equals(t, "1.2.3.4", host)
testutil.Equals(t, "http", port)

host, port, err = net.SplitHostPort(":1234")
testutil.Ok(t, err)
testutil.Equals(t, "", host)
testutil.Equals(t, "1234", port)

host, port, err = net.SplitHostPort("yolo")
testutil.NotOk(t, err)

Better 🤓

for _, tcase := range []struct{
    name string

    input     string

    expectedHost string
    expectedPort string
    expectedErr error
}{
    {
        name: "host and port",

        input:     "1.2.3.4:1234",
        expectedHost: "1.2.3.4",
        expectedPort: "1234",
    },
    {
        name: "host and named port",

        input:     "1.2.3.4:something",
        expectedHost: "1.2.3.4",
        expectedPort: "something",
    },
    {
        name: "just port",

        input:     ":1234",
        expectedHost: "",
        expectedPort: "1234",
    },
    {
        name: "not valid hostport",

        input:     "yolo",
        expectedErr: errors.New("<exact error>")
    },
}{
    t.Run(tcase.name, func(t *testing.T) {
        host, port, err := net.SplitHostPort(tcase.input)
        if tcase.expectedErr != nil {
            testutil.NotOk(t, err)
            testutil.Equals(t, tcase.expectedErr, err)
            return
        }
        testutil.Ok(t, err)
        testutil.Equals(t, tcase.expectedHost, host)
        testutil.Equals(t, tcase.expectedPort, port)
    })
}

Tests for Packages / Structs That Involve `time` package.

Avoid unit testing based on real-time. Always try to mock time that is used within struct by using, for example, a timeNow func() time.Time field. For production code, you can initialize the field with time.Now. For test code, you can set a custom time that will be used by the struct.

Avoid 🔥

func (s *SomeType) IsExpired(created time.Time) bool {
    // Code is hardly testable.
    return time.Since(created) >= s.expiryDuration
}

Better 🤓

func (s *SomeType) IsExpired(created time.Time) bool {
    // s.timeNow is time.Now on production, mocked in tests.
    return created.Add(s.expiryDuration).Before(s.timeNow())
}

Enforced by Linters

This is the list of rules we ensure automatically. This section is for those who are curious why such linting rules were added or want similar ones in their Go project. 🤗

Avoid Prints

Never use print. Always use a passed go-kit/log.Logger.

Ensured here.

Ensure Prometheus Metric Registration

It's very easy to forget to add Prometheus metrics (e.g a prometheus.Counter) into a registry.MustRegister function. To avoid this, we ensure all metrics are created via promtest.With(r).New* and we disallow the old type of registration. Read more about the problem here.

Ensured here.

go vet

Standard Go vet is quite strict, but for a good reason. Always vet your Go code!

Ensured here.

golangci-lint

golangci-lint is an amazing tool that allows running a set of different custom linters from the Go community against your code. Give it a star and use it. (:

Ensured here with those linters enabled.

misspell

Misspell is amazing, it catches typos in comments and docs.

No Grammarly plugin for this yet ): (We wish).

Ensured here, using golangci-lint / misspell.

Comments Should be Full Sentences

All comments should be full sentences. They should start with an uppercase letter and end with a period.

Ensured here using golangci-lint / godot.

Bash

Overall try to NOT use bash. For scripts longer than 30 lines, consider writing it in Go as we did here.

If you have to, we follow the Google Shell style guide: https://google.github.io/styleguide/shellguide.html Ensured here using shfmt. We also use shellcheck to check any script errors.

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Thanos Coding Style Guide

Go

Development / Code Review

Reliability

Defers: Don't Forget to Check Returned Errors

Exhaust Readers

Avoid Globals

Never Use Panics

Avoid Using the reflect or unsafe Packages

Avoid variable shadowing

Performance

Pre-allocating Slices and Maps

Reuse arrays

Readability

Keep the Interface Narrow; Avoid Shallow Functions

Use Named Return Parameters Carefully

Clean Defer Only if Function Fails

Explicitly Handle Returned Errors

Avoid Defining Variables Used Only Once.

Only Two Ways of Formatting Functions/Methods

Control Structure: Prefer early returns and avoid else

Wrap Errors for More Context; Don't Repeat "failed ..." There.

Use the Blank Identifier _

Rules for Log Messages

Comment Necessary Surprises

Testing

Table Tests

Tests for Packages / Structs That Involve time package.

Enforced by Linters

Avoid Prints

Ensure Prometheus Metric Registration

go vet

golangci-lint

misspell

Comments Should be Full Sentences

Bash

Avoid Using the `reflect` or `unsafe` Packages

Control Structure: Prefer early returns and avoid `else`

Use the Blank Identifier `_`

Tests for Packages / Structs That Involve `time` package.